Liftable auxiliary axle assemblies have frequently been used on vehicles to increase the load carrying capacity of the vehicle without increasing the maximum permissible load on each wheel and tire assembly of the vehicle as regulated by state law. Most of these liftable auxiliary axle assemblies have been limited to the vehicle prime mover section such as the tractor of a semi-trailer rig or a fixed bed truck and have not been practically applicable to truck trailers, especially semi-trailers. While several of the prior art patents such as U.S. Pat. No. 3,689,102 illustrate a liftable auxiliary axle assembly applied to a semi-trailer, these illustrated applications have met with virtually no practical commercial success. This lack of practical commercial success with liftable auxiliary axles is due in large part to the physical geometry of a semi-trailer/tractor connection which will subject the liftable auxiliary axle assembly to excessive size loading.
As a result of these problems, semi-trailers have by and large been relegated to the use of the typical non-liftable tandem axle suspensions as the trailer suspension system. While these tandem axle suspension systems are usually required when the semi-trailer is loaded, they are generally not required when the trailer is empty. Because these semi-trailers have not used non-liftable tandem axle suspension, unnecessary tire wear has been encountered on the tires of at least one of the axles of the tandom axle suspension. Further, because of the geometry of the connection between the semi-trailer and the tractor pulling it, both the trailer frame and the tandem axle suspension were subjected to relatively high side loads all the time the trailer is being operated regardless of whether it is loaded or empty. This again reduces the fatigue life of the material used in the fabrication of the trailer frame and the tandem suspension.